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Alarm Bag 


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build, hack, tweak, share, discover,- 

Alarm Bag 

Written By: Steve Hobley 


Scissors (1) 

Soldering iron (1) 

hand-held electric drill and 1/8" bit (1) 


12v DC Piezo Siren (1) 
from RadioShack. 

Panel Mount Mono 1/8" Phone Jack (2) 
from RadioShack. Note that this is a 
headphone jack, not a telephone jack. 

6-ft Audio Cable (1) 
from RadioShack. 

■ • 

8 "AA" Battery holder (1) 
from RadioShack. 

Batteries (8) 

from RadioShack. 4 pack, need two 


Battery Snap Connectors (1) 
from RadioShack. 

1/4" plywood and a scrap of 1/8" clear 
plastic (1) 

A messenger-style shoulder bag (1) 
Or any other sort of hand or shoulder 

Heat-shrink tubing, multicolor 

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Alarm Bag 

assortment (1) 
from RadioShack. 

Jewelry wrapping wire (1) 


This project uses simple electronics to create an alarm circuit for a shoulder bag. The key 
components are two 1/8" headphone jacks with "normally closed" switches, which detect 
whether a plug is in the jack. Wiring two of these jacks in series with an alarm creates a 
circuit that activates when both plugs are removed. With this arrangement, you can use one 
jack to keep the alarm from sounding while you arm the system. The other jack accepts a 
plug that you can tether to a belt loop or other anchor. When the bag is pulled away from its 
anchor plug, the alarm sounds. You can of course use this same circuit for a handbag, 
backpack, or any other personal cargo carrier. 

Check out more Weekend Projects . 

Step 1 — Assemble the Parts 

• We're going to make a simple 
circuit from the battery clip, 
through to the two audio jacks, and 
finally, attaching to the siren itself. 

• The idea is that the alarm will 
sound when both audio plugs are 
removed from the jacks - we use 
two so that the alarm can be safely 
"deactivated" while the trigger 
cable is removed from whatever it 
is attached to. 

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Alarm Bag 

Step 2 — Wiring the Parts 

• This circuit is very simple. Using just the red and black wires attached to the siren and the 
battery clip, we're going to put all the components in series. 

• The sockets have a "normally closed" switch built right in. We will be using this as our 
alarm trigger. Look at the pictures and you will see the 2 (of 3) terminals we connected to 
take advantage of the switch. 

• When both plugs are removed, the circuit will be completed and the alarm will sound. 

Step 3 — Preparing the Trigger Cable 

• Additionally, we need to cut the audio cable in two, with one part longer than the other. I 
created a small loop in the end of the long part with jewelry wire and heat shrink. 

• Put the heat shrink on BEFORE you wrap the wire! 


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Alarm Bag 

Step 4 — Making the Enclosure 

• Originally I was going to use a 
plastic enclosure, but the batteries 
wouldn't fit inside, and the next size 
up was too big. Instead, I made a 
small custom case from plywood 
that fit snugly around all the parts. 

• To create a speaker hole to the 
outside, drill equidistant holes in a 
piece of clear plastic with a 1/8" bit. 

Step 5 — Installing in the Bag 








5 ^^^ 

• To fit the enclosure into the bag, we first need to place it inside, up against the side wall. I 
drilled two small holes to fit the front plate. 

• Using this as a guide, I cut out a 1" circular hole for the speaker and replaced the front 

• Finally, drill a hole in the side and install one jack. This is where the trigger cable will be 
placed when the device is armed. 

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Alarm Bag 

Step 6 — Testing! 

Hft\ ^B 

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• First thing is to make sure the system is disarmed by installing a plug into the jack inside 
the bag. 

• Next the battery clip can be connected. This is the "unarmed" state. 

• When you want to arm the system, attach the long cable around the "home" anchor that 
you want the bag to remain near - this could be your wrist, your belt, a bike pannier, or 
table leg. 

• Plug in the external jack first, then remove the internal one. 

• Now the siren is armed. Anyone snatching the bag will disconnect the output plug and 
trigger the alarm. 

• To disarm, just install the internal plug, and the external one can be safely removed. 

• One final tip: In time, you might find that the external jack pulls free from the bag 
wall. I used a larger washer on the socket and that seemed to fix it. 

This project uses some common components in a slightly unorthodox way to produce something 
completely new -- specifically the switching action of audio sockets. This is one of the principals 
of invention -- the combination of old stuff in a different way to make new stuff. 

A possible extension to this project would be to make a light-controlled switch with a 555 timer 
and a transistor. So when the bag is opened, the alarm would sound for a fixed period. 

This document was last generated on 201 2-1 1 -02 02:1 1 :1 2 PM. 

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